Autorotation

Autorotations are used to perform power off landings from altitude in the
event of an engine failure. A JPEG or
GIF sequence of
photographs showing an autorotation is available.

Maneuver Description

An autorotation is used when the engine fails, or when a tail rotor failure
requires the pilot to effectively shut down the engine. It is very similar
to gliding in an airplane.

The entry

To enter the autorotation, the pilot lowers collective all the way down,
simultaneously adding right pedal. Lowering the collective maintains RPM
during the entry to autorotation, and keeps the AOA (angle of attack) at
a normal value during the glide.

Adding the right pedal is necessary because in autorotation there is no
torque. During power-on flight, the pilot was using a lot of left pedal
to counter the torque being produced by the engine. Once the helicopter
is autorotating, the engine disengages and produces no more torque.

While the collective is being lowered, the nose of the helicopter has a
tendancy to pitch down. The pilot needs to use aft cyclic to prevent
this. Allowing the nose to pitch down creates two problems: it tends to
reduce RPM because it decreases the amount of airflow through the rotor
disk, and it tends to increase airspeed, usually far above the range you
want to use while autorotating.

Establishing the glide

As the air starts flowing up through the rotor system, the RPM will start
to increase, and depending on how the helicopter is rigged, the RPM may
get too high. In this case, as RPM gets high the pilot can increase
collective pitch to lower RPM.

The pilot should set up a normal autorotational attitude in order to get
a normal airspeed. Although helicopters will autorotate at zero airspeed
and even at negative airspeed, normally the pilot will want to hold between
60-70 knots of airspeed during the glide.

Selecting a landing area

Hopefully within the first few seconds the pilot will establish autorotation
and will have selected a landing area. The approach to the landing should
almost always be into the wind, so the pilot needs to select a spot
which will allow him to maneuver for an upwind approach.

The spot should normally be flat, firm, and fairly level. A spot like this
is not always reachable, but is obviously preferred.

One thing I quickly look for is poles which may have wires strung. If I
have any other place to land, I'll stay away from one which may
have wires. The last thing I need to be doing on short final is trying
to duck wires!

Once the pilot has selected a landing area, I recommend he visualize a
standard traffic pattern imposed on the landing area and aligned with
the wind. The pilot should figure out which leg he is currently on,
and then fly the pattern so that he arrives on final approach at an
altitude and airspeed which will allow him to land in the selected area.

By flying a rectangular traffic pattern, the pilot can find himself on
base leg, watching the angle to the landing area. When the angle is right,
he simply turns final and will be very close to the desired spot. If the
pilot starts to see the angle before he reaches the extended "centerline",
he can simply turn final early. By cutting the corner he reduces the
distance he has to fly, and makes it to the spot without ending up too
low.

If the pilot finds himself slightly high on base, he can simply fly
through the extended centerline, and turn a little late onto final.
The extra distance uses up some extra altitude, and he still makes it
to his spot.

A little overshoot is preferable to a little undershoot because it can
be corrected easilly still leaving sufficient energy. An undershoot
normally requires going to best glide airspeed and dragging the rotor
RPM down to the lowest allowable value. If the pilot is not careful,
the result may be reaching the spot with low RPM. This is probably not
a problem with a light inertia rotor system, but in a high inertia
rotor system the RPM might not be recovered before touchdown.

The Flare

The pilot initiates the flare by using aft cyclic. No collective or pedal
input is normally required. The height that the pilot should start to
flare at depends on many factors, including the model helicopter, the
descent rate, the airspeed, the descent rate, the headwind component,
and how rapidly the pilot is going to move the cyclic.

The purpose of the flare is twofold. First, it slows the descent rate
of the helicopter, from 1,000 or 2,000 feet per minute to much less,
so that a soft touchdown can be made. It also reduces the forward ground
speed to just a few knots (we hope!) so that sliding on the landing gear
is minimized.

The flare must be timed to not zero the descent rate, because the helicopter
would be left hanging in the air bleeding RPM, but rather the flare should
be timed to slow the descent rate so that the helicopter is approaching the
ground at a managable rate. The descent rate should be decreasing so that
it either goes to zero just above the ground, or is low enough that a little
collective pitch can bring it to zero.

The Landing

Touchdown is accomplished by (typically) putting the helicopter into a level
attitude, and then using the collective to cushion the landing, just as in
a hovering autorotation. The pedals are used to align the landing gear with
the ground track.

Power Recovery

If the pilot is practicing an autorotation he may decide to recover to a
hover, rather than touch down. The procedure is to start raising collective
while still in the flare, just as flare effectiveness starts to go away,
before any increase in sink rate is experienced. By starting the recovery
early, the engine is not trying to play catch-up, and the recovery can be
made with the RPM in the green range at all times.

Common Mistakes

Gosh there are a lot! Here are a few:

Failure to Lower Collective all the way down

If the pilot forgets to lower collective and this is a real engine failure,
it's a fatal mistake. Lowering collective is the most important part of
doing an autorotation. If you remember to do that, you will probably walk
away from the landing. Some pilots only put the collective pitch part of
the way down. They get to "know" where it belongs. The only problem with
this is that the position the collective needs to go to depends on many
factors such as pitch link rigging, gross weight, and density altitude.
These things can change from day to day. This method also delays recovery
of rotor RPM, and there is no good reason to do that.

The best method is to lower collective all the way, and as RPM starts to
build back up some collective should be raised to stop the RPM somewhere
in the operating range.

Failure to trim with anti-torque pedals

Pilots will either forget to push right pedal, or push too much, or even
sometimes push the left pedal! In any case, the aircraft should be
autorotated in trim, and the pilot can do this by putting in the correct
amount of right pedal when the engine fails.

Allowing the nose to drop

We already discussed it, but I'm repeating this because it's one of the
most common errors I see during the entry. Do not let the nose
drop during the entry. Whatever attitude the helicopter is in, enter
the autorotation in that attitude, and then after the autorotation is
established the pilot can make any attitude adjustments required for
proper airspeed. Allowing the nose to pitch down delays the recovery of
RPM (it's like an anti-flare) plus it is not uncommon for pilots to
overspeed the rotor by waiting until the airspeed builds to 80 knots or
more, and then suddenly trying to fix it by yanking back on cyclic. The
result is an almost instantaneous rotor overspeed.

Failure to control Rotor RPM with collective

Most helicopters are rigged so that at normal weights the collective will have
to be raised somewhat to keep rotor RPM in the normal operating area. Common
mistakes are either to leave the collective full down so long that a rotor
overspeed occurs, or to overcontrol the collective, moving it up and down
during the entire glide. The proper way to manipulate collective is to lower
it full down during the entry to autorotation. Then, as RPM starts to increase
toward the normal operating area raise enough collective to stop the RPM
from changing. Wait a few seconds until it stabilizes, and make one final
adjustment to place the RPM exactly where it is desired. Normally no further
manipulation of the collective will be required during the glide. One
exception is that during turns, especially at high speed, some collective
may be required to prevent the RPM from climbing too high. Rolling out of
the turn, the pilot should put the collective back to where it was before
the turn was entered. By performing turns at lower airspeeds, little or no
collective will be required.

Failure to maneuver to the point of intended landing

Many pilots get quite proficient at autorotating to the runway at their
home airport, but have more trouble when trying to make a specific landing
area in the off airport environment. I advocate setting up a (tight) traffic
pattern to the landing area, just as is done at an airport. The pilot
should figure out the wind, and therefore where "final" will be. Then the
pilot should figure out where he currently is with respect to the traffic
pattern (is he already on downwind, base, or final?). Once he knows what
leg he is on, he can manipulate the length of the remaining legs to arrive
on final at the proper altitude. I also suggest a very short final. The
longer final is, the bigger the chance is of over or undershooting, with
no easy way to correct once the under or overshoot is recognized. Instead,
fly a very tight base and time your turn onto short final to give you the
desired distance to the touchdown spot. If you are a little low, turn final
slightly early. If you are a little high, delay the turn to final, overshoot
the centerline somewhat, and use up the additional altitude on base. For
gross errors, S-turns or zero (or negative) airspeed may be required. One
final rule I have is never do a 360 degree turn. You lose track of
your approach angle for too long. Instead, if you have massive amounts of
altitude to lose, perform a figure-8 pattern on final. This way the spot is
always visable, and you can turn back onto final when the angle begins to
look right.

Flaring at the wrong altitude

Each helicopter has a range of altitudes it needs to be flared at. The
altitude will change from flight to flight based on gross weight, density
altitude, wind, and airspeed. Generally, aircraft with higher disk
loadings require a higher flare. If the pilot flares too high, the
helicopter will stop it's descent too high above the ground to make a safe
landing. If the pilot flares too low, he will be forced to level the
helicopter (get rid of the flare) too early (to avoid hitting the tail on
the ground). The result will be a high rate of descent (which he can
probably fix by raising collective) and high forward ground speed (which
he can't fix, so he'll slide hundreds of feet).

Assuming we can't always make a perfect flare, which way we would rather err
depends on the surface we are going to land on. If the surface is firm and
level, some slide probably won't hurt, and we'd rather be a little bit low
so we get a nice soft touchdown, followed by a little slide. If the surface
does not appear to allow us to slide (swamp or such which will cause the skids
to dig in) the flare should probably be a little high to insure we can get
rid of all forward speed. We may touch down a little harder, but by being
more vertical we reduce the chance of rolling over. One caveat is that human
beings do not take vertical accelerations well at all, so if people are going
to avoid back injuries, the flare better not be too high.

Flaring too aggressively or not aggressively enough

The speed with which the nose of the aircraft needs to be pitched up is
related to gross weight, density altitude, wind, and airspeed. Generally
if gross weight is high, a more aggressive flare will be required. If
density altitude is high, a more aggressive flare is required. If wind
is high, a less aggressive flare is required. And if airspeed is
high, a less aggressive flare is required. Pilots can adjust for minor
airspeed deviations by flaring at different altitudes, or with different
amounts of aggressiveness. For instance, if the airspeed is 10 knots below
optimal, a more aggressive flare will help to make up for this. Of course
there are limits to the amount of correction that is possible.

Failure to level the aircraft

Some aircraft land in a slightly tail low attitude, but with many others
it is critical to have the landing gear level before touchdown. Failure
to do so can result in tail boom strikes and porpoising (where you hit on
the heels, and then roll up onto the toes and flip over forward).

Failure to maintain heading during the slide

There are a couple reasons that heading might not be maintained during
any ground slide. One is just that the pilot fails to manipulate the
pedals correctly, the other is that if rotor RPM gets too low the tail
rotor may lose effectiveness. Failure to maintain heading can cause a
skid gear to catch and roll the aircraft over on it's side. Most aircraft
can perform fairly high speed slides if the skids are pointed in the direction
the aircraft is moving.

Moving the cyclic aft during the slide

It's human nature to want to stop the slide as early as possible, but moving
the cyclic aft has two problems. One is that the main rotor is probably not
generating much thrust at this point, so it won't help much anyway. The other
is that flapping is at maximum because RPM is low, and moving the cyclic
aft moves the rotor blades even closer to the tailboom. The rotor blades
hitting the tailboom is a very real possibility.
Paul Cantrellpaul at copters.com
(replace " at " with "@" to email me - this avoids SPAMMERS I hope)